The present application relates to optical networking and, more particularly, in certain embodiments, to systems and methods for determining the status of an optical communication link.
In addition to providing high-speed data communication between remote points, modern optical networks are also expected to provide a very high degree of reliability. To provide this reliability, many optical protection mechanisms have been developed, such that a failed channel or even all the channels may be sent down an alternate “protection” path in the event of a failure. The action taken in diverting traffic in this way is referred to as protection switching.
Before protection switching can occur in response to a failure, the failure must be detected. One important attribute of a failure detection scheme is that it should be very fast. The time taken to detect a failure is part of the overall time to switch in response to the failure. If the total protection switchover time is too long, a significant amount of data will be lost and the user experience will be affected. The failure detection mechanism should also be very reliable. No failure should be missed and there should be no false indications of failure. It is difficult to maintain this reliability in the presence of optical noise as exists in real-world optical communication links since crude analysis may mistake the noise for the desired signal. It is also desirable that a failure detection method be adaptable for both multi-channel and single-channel use.
Numerous failure detection mechanisms have been developed. Perhaps the simplest scheme is to use photodiodes to measure optical power. If optical power is lost then there is deemed to be a failure. This technique, however, is not able to differentiate optical noise, e.g., amplified spontaneous emission (ASE) noise, from the signal content of optical channels. This drawback is particularly problematic when one considers that modern optical communication links include cascaded optical amplifiers. Optical amplifiers, such as Erbium-doped fiber amplifiers (EDFAs), emit optical noise power in the form of ASE noise, noise that is elevated when their input power is lost due to a fiber cut. Such noise may be further amplified by successive amplification stages. A simple power measurement technique is thus unworkable for failure detection.
Another class of optical failure detection methods relies on optical spectrum analysis. In one implementation of this type of analysis, a monitor signal is tapped off for input to an optical spectrum analyzer (OSA). When the channels known to be present at the input are all missing from a spectrum as developed by such an analyzer, a failure is determined. Alternatively, individual monitors may be coupled to each optical channel following the demultiplexing stage of a link receiver. Such monitors are also, however, affected by ASE noise that passes through the demultiplexer. Furthermore, the spectral analysis techniques are slow and require expensive equipment.
Other techniques rely on monitoring of the recovered data across the multiple channels. A fiber cut can be determined when an alarm indication is generated on the receivers for each of the channels signifying loss of data reception. One can also rely on special overhead information that is then detected by photodiodes and dedicated circuitry. These techniques require specialized electronic equipment.
Systems and methods for reliably and quickly detecting failures in an optical communication link are needed.